In a number of non-destructing systems, balancing is necessary to obtain accurate indications and measurements, the desired balance being usually established when a null indication is obtained in a particular condition of probe means relative to a part. For example, in eddy current systems in which a test coil is in one leg of a bridge circuit or in which a test coil and a reference coil are inductively coupled to a primary winding and connected to develop a differential output signal, it may be desired to obtain a null indication when the test coil is placed against or in predetermined relation to a reference part of known characteristics, or when the test coil and a reference coil are in the same relationship to two parts or two portions of the same part which have the same characteristics. To obtain a balance, it has been the practice to provide manually adjustable impedances and/or arrangements using manually adjustable phase shifters for injecting an AC signal of the proper magnitude and phase. The adjustment of such systems has been difficult and time-consuming and frequent readjustment is oftentimes necessary to offset drift from heat, conductivity changes, permeability changes, etc.
Arrangements have heretofore been proposed for automatic balancing of such systems, using servo motors for adjustment of potentiometers. An electronic circuit arrangement is proposed in the Mori et al. U.S. Pat. No. 3,566,258 using synchronous rectifier circuits coupled to the output of a bridge circuit and to an AC source with supplementary AC signals being supplied to a variable gain amplifier connected to the outputs of the synchronous rectifier circuits. The operation is not entirely clear but it appears that, if operable at all, the system would be limited to the detection of rapid changes, such as produced by movement of the test coil over flaws, the outputs of the rectifier circuits being applied directly to the variable gain amplifier circuits. One mention is made as to limiting the response speed as to a first rectifier and reducing the time constant of the second, but such would not be very satisfactory in many applications. For example, it is oftentimes desirable when testing with respect to characteristics other than flaws, such as changes in permeability or conductivity in which the response time is generally slow, to obtain a balance only when a test coil or probe is in a predetermined relationship to a part of known characteristics. In such cases, continuous and rapid automatic balancing would not permit accurate testing. The Mori et al. patent system also requires a substantial number of phase shifters and, more important, is limited to the use of a bridge circuit to which the signals are applied.